1. Field of the Invention
The invention relates to a valve, in particular a control valve in a fuel line of an injection system for internal combustion engines.
2. Description of Prior Developments
DE 4334802 C1 has disclosed a control valve which is arranged between an injection pump and a valve that opens under high pressure, in the injection line in fuel injection systems for diesel engines. In this arrangement, the known control valve controls the valve cross section of a return branching off from the injection line. The control valve has a valve tappet which is arranged in a longitudinally displaceable manner and which exposes larger valve cross sections of the return as the opening stroke increases and can be actuated against the restoring force of a valve spring by an electric magnet coil. With the magnet coil deenergized, the valve spring holds the valve in the open position, with the result that the fuel supplied flows off via the return. If the valve tappet closes the return owing to the magnetic force of the coil, the pressure required to trigger the injection process is built up rapidly in the injection line. The injection valve at the end of the injection line opens when the rated pressure is reached and allows access to the combustion chamber.
To allow precise metering of extremely small injection quantities at high engine speeds, the valve cross section should be adjustable. One known solution of this problem is to provide a spring assembly having at least two actuating springs which can be deflected in the axial direction of the valve tappet and which are arranged in parallel and in a manner which allows them to be subjected to a force in an axially offset manner by the valve tappet. If voltage is applied to the magnet coil, the valve reduces the cross section of the return against the force of the first spring in a first stroke range. If the voltage at the magnet coil is increased until the valve has passed through the first stroke range in the closed position, the valve tappet comes into engagement with the second spring, which counteracts the closing movement of the valve in a second stroke range in addition to the first spring. If the voltage is increased further, the valve comes to rest against the valve seat after passing through both stroke ranges counter to the resultant restoring force of both springs, with the result that the return is closed. Owing to possible fluctuations in the drive voltage of the coil and the resulting interaction between the valve tappet, which is moved in free oscillation, and the spring forces, it is not possible to set accurate positions of the closing member nor to set precise valve cross sections.
DE 3700356 A1 proposes a similar solenoid valve for controlling injection systems, the electromagnet of which has two switchable magnetic forces for three corresponding positions of the valve tappet. In this known control valve too, the valve tappet is assigned two return springs, of which the first spring engages continuously on the valve tappet and the second spring comes into operative engagement during displacement, in the stroke range between the completely open position and the intermediate position.
In the case of the known control valves for the fuel return in the injection line of internal combustion engines, the entire magnet and hence valve stroke must be designed in such a way that the valve cross section exposed is sufficiently large to ensure reliable replenishment of the fuel removed during injection in the time between injection events at all engine speeds. During the relief process at the end of the respective injection, the valve cross section is too large, particularly at low engine speeds, resulting in overrelief of the high-pressure space. It has been found that it is not possible accurately to set the valve cross sections required to avoid overrelief with the valves of the known type, especially in the intermediate positions. This results in instabilities in the injection process envisaged. Indeed, if a fuel injection involving a plurality of partial injection processes during the operating cycle is envisaged, the overrelief due to the cutting off of the pilot injection considerably disrupts or totally prevents the main injection.